Atomic StructureChapter 4.1

The Nature of Scientific Models

o Tend to change over time:

Problems are discovered and explored• New information becomes available

• New materials become available

• New technology becomes available

o Older models are often modified

o Sometimes old models totally discarded because proved to be just plain wrong

o Studying the structure of atoms is a little like studying wind.

o Because you cannot see air, you must use indirect evidence to tell the direction of the wind.

o Atoms pose a similar problem because they are extremely small.

Even with a microscope, scientists cannot see the structure of an atom with “visible” light.

o Cut a piece of aluminum foil in half

• Two smaller pieces, same shiny, flexible substance.

o Cut the pieces again, and again.

• Can you keep dividing the aluminum pieces forever?

o Greek philosophers debated a similar question about 2500 years ago.

Ancient Greek Models of Atoms

Democritus (Greek philosopher circa 400 BCE):

Posits matter is extremely small particles that can not be divided.

oNo hard physical proof, logical deduction only.

oCalled these particles “atoms” from the Greek word atomos, which means “uncut” or “indivisible.”

Ancient Greek Models of Atoms

Aristotle (Greek philosopher circa 350 BCE)

Posits everything made from four elements:

earth, air, fire, water. oNo hard physical proof, logical deduction

only.oDid not think there was a limit

to the division of matter.

• For centuries people accepted

Aristotle’s views.

• In 1800s, scientists had

experimental data to

support an atomic model.

1. Dalton studied the behavior of gasses in air.

• Based on the way gasses exert pressure, Dalton concluded (correctly) that:

A gas consists of individual particles.

2. Dalton measured masses of elements that combine when compounds form.

Ratio of element masses was always the same in each compound.

• In other words, compounds have a fixed composition.

John Dalton’s Evidence for Atoms

Example of element ratio:

When magnesium burns, it combines with oxygen.

Ratio of the mass of magnesium to the mass of oxygen is always about 3 : 2

Magnesium dioxide has a fixed composition.

Dalton’s Atomic Theory

1. All elements composed of atoms• atoms are tiny solid spheres

2. All atoms of same element have same mass• Atoms of different elements have

different masses.

3. Compounds contain more than one element.

4. In a compound, atoms of different elements always combine in fixed ratio.

Dalton’s Atomic Theory 1803

Dalton made wooden spheres as a model to represent the atoms of different elements.

• A tiny, solid sphere with a different mass represents each type of atom.

Dalton’s Atomic Theory

• Theories must explain data from many experiments. Because Dalton’s atomic theory met that

goal, the theory became widely accepted.

• We now know not all of Dalton’s ideas about atoms were completely correct. Over time, scientists learned more and

revised the theory to take into account new discoveries.

Dalton’s Atomic Theory

J.J. Thomsono First to find evidence that atoms

are made of even smaller particles.

Thomson’s Model of the Atom:o When some materials are rubbed,

they gain the ability to attract or repel other materials. Such materials are said to have either a

positive or a negative electric charge. •Objects with like charges repel, or push apart. •Objects with opposite charges attract, or pull together.

o Amber is the hardened form of sap, a sticky, viscous liquid that protects trees from insects and disease. If amber is rubbed

with wool, it becomes charged and can attract a feather.

Thomson’s Model of the Atom

Thomson’s Experiments

In his experiments, J. J. Thomson used a sealed tube containing a very small amount of gas.

Sealed tube filled with gas at low pressure

Glowing beam

Metal disk Source of electric current

Metal disk

Metal disk Source of electric current

Positive plate

Negative plate

When the current was turned on, the disks became charged, and a glowing beam appeared in the tube.

•Observed:

oBeam repelled by the negatively charged plate

oBeam attracted by the positively charged plate.

•Hypothesized: oParticle interaction with air in the tube caused

the air to glow.

oBeam was a stream of charged particles

Evidence for Subatomic Particles

oConcluded particles in beam had negative charge

attracted to positive plate

oHypothesized particles came from inside atoms because:

For all different metals Thomson used for the disk, the particles were same.

Particles had very, very low mass

~ 1/2000 the mass of a hydrogen atom, the lightest atom.

o Thomson revised Dalton’s model to account for subatomic particles. Dalton: atoms are tiny solid spheres

Thomson’s Model of the Atom (~1897)

o Overall atom is neutral:

has a positively charged bulk

negative charges evenly scattered throughout

• Thomson’s model was called the “plum pudding” model. Today, it might be called the “chocolate chip ice cream” model.

The chips represent negatively charged particles, which are spread evenly through a mass of positively charged matter—the vanilla ice cream.

Thomson’s Model of the Atom

o Ernest Rutherford designed an experiment to find out what happens to alpha particles when they pass through a thin sheet of gold. Alpha particles: fast-moving, positively charged

particles.

o Rutherford hypothesized (based on Thomson’s model):

Mass and charge at any location in the gold would be too small to change the path of an alpha particle.

He predicted that most particles would travel in a straight path

Rutherford’s Atomic Experiment

The Gold Foil ExperimentParticles light up screen

when they hit it

Beam of alpha particles

Source of alpha particles

Slit

Deflected particle

Undeflected particle

Screen

Gold atoms

Alpha particles

Alpha particles

Nucleus

Expected

Observed

Discovery of the Nucleus

oThe alpha particles whose paths were deflected must have come close to another charged object. The closer they came, the greater the deflection.

oHowever, many alpha particles passed through the gold without being deflected. These particles did not pass close to a charged object.

Rutherford’s Atomic Theory

o Thomson’s model did not explain all of the evidence from Rutherford's experiment.

Rutherford proposed a new model:• The positive charge of an atom is not

evenly spread throughout the atom.

• Positive charge is concentrated in a very small, central area.

• The nucleus of the atom is a dense, positively charged mass located in the center of the atom.

Rutherford’s Atomic Theory

o The Houston Astrodome: 9 acres and 60,000 seats.

If the stadium were a model for an atom, a marble could represent its nucleus.

Total volume of an atom is about a trillion (1012) times the volume of its nucleus.

Rutherford’s Atomic Theory

Scientists and the Atom Max Planck - In 1900, he produced the idea that radiation

is emitted in discrete quantities which he later called quanta.

Albert Einstein – In 1905, building on Planck's ideas he published the idea that the "quanta" was a bundle of light that behaved like a particle.

Ernest Rutherford – In 1911, working with his colleagues Hans Geiger and Ernest Marsden, Rutherford became the first individual to hypothesize that the center of the atom was small, dense, and positively charged. He designated this area as the nucleus.

Scientists and the Atom

James Chadwick: In 1921, while working with E.S. Bieler, he proposed that the nucleus was held together by a strong force. In 1932, he proposed that the nucleus contained both neutrons and protons.

Niels Bohr: In 1913, he proposed a model of the atom with electrons orbiting the nucleus similar to the planets revolving around the sun. He determined that orbits of electrons depend on their energy, and that electrons can jump from one energy level to another; energy travels in discrete quantities.

Louis de Broglie: In 1924, he proposed that electrons could behave as waves under some conditions. This helped scientists determine that the atom didn't behave like the solar system because electrons do not move in regular orbits.

Scientists and the Atom

Erwin Schrodinger: In 1926, he developed the basic equation of quantum mechanics building on de Broglie's concept that electrons behave like waves in some situations.

Werner Heisenberg: In 1927, he suggested that it is impossible to determine the position and velocity of an electron at the same time; this concept came to be known as the uncertainty principle.

Max Born: In 1927, working with Heisenberg, Born modified Schrodinger's equation of quantum mechanics. His idea enabled scientists to develop a model of an atom displaying the nucleus surrounded by electrons at varying locations when they are in diverse energy states.

Summary

1. Democritus (400 BCE) – matter made of atoms.

2. John Dalton (1803) – proof for atoms.3. J.J. Thomson (1897) – discovered electrons;

plum pudding model positive sphere with electrons embedded in it.

4. Ernest Rutherford (1914) – discovered nucleus; proposed that electrons orbited a nucleus that had almost all the mass.

5. Niels Bohr (1913) – electrons orbit in fixed paths.

6. Werner Heisenberg (1926) – Electrons move about the nucleus in a cloud.

7. James Chadwick (1932) – nucleus contained neutrons as well as protons.

Assessment Questions

1. The ancient Greek philosopher Democritus coined what word for a tiny piece of matter that cannot be divided?

a) Element

b) Electron

c) Atom

d) Molecule

Assessment Questions

2. Dalton’s theory (~1800; based on behavior of gasses) included all but one of the following points. Which is not from Dalton?

a) All elements are composed of atoms.

b) Most of an atom’s mass is in its nucleus.

c) Compounds contain atoms of more than one element.

d) In a specific compound, atoms of different elements always combine in the same way.

Assessment Questions3. J. J. Thomson’s gas tube electrical

experiments (~1897) provided first evidence of: a) Atoms

b) Nucleus

c) Subatomic particles

d) Elements

Assessment Questions

4. Rutherford’s gold foil experiment (~1914) provided evidence for one of these:

a) Negative and positive charges are spread evenly throughout an atom.

b) Alpha particles have a positive charge.

c) Gold is not as dense as previously thought.

d) There is a dense, positively charged mass in the center of an atom.